Hydrogen is the most abundant of all elements and is an important ingredient in many industrial processes. Hydrogen also has potential to be a synthetic fuel for a variety of purposes traditionally served by petroleum. One of the chief difficulties, however, in using hydrogen for present or future fuel purposes is its storage in high pressure or in insulated tanks. As the least dense substance known, hydrogen requires large volumes, high pressures (such as 2000 psi) or both when conventionally stored. Regardless of how much pressure is applied, hydrogen will not liquify as will some gases, e.g., propane. Hydrogen only becomes a liquid at temperatures below -253.degree. C. (-423.degree. F.) and in order to contain this ultra-cold liquid for any reasonable period of time, super-insulated containers are required.
Hydrogen at low pressures can also be stored compactly in metal containers at moderate pressures and normal temperatures when it is chemically absorbed into certain powdered sorbent alloys which are conventionally known to form "metal hydrides". Typically, these powdered hydrides are less than five microns in size and are highly irregular in shape being jagged and having crevices.
These powdered materials, e.g., FeTi or LaNi.sub.5 absorb (through an exothermic process) and release (through an endothermic process) hydrogen gas at pressures typically less than 1 MPa (150 psi) and at normal room temperatures, e.g., 20.degree. C. (68.degree. F.). During the absorption process the irregular shaped sorbent alloys expand by about 15-25% in volume, in much the same way as water changing to ice expands, and the alloys are capable of straining and bursting any container, by exerting solid pressures of at least 5000 psi, unless the container is designed to allow for this type of expansion. The stress from the metal hydrides is cumulative and grows over a number of absorption-desorption cycles to first deform, then bulge, and to finally burst the container. When this occurs a dangerous condition arises through the expulsion of fine pyrophoric powders in a flammable gas.
A metal hydride container usually incorporates means for rapidly transferring heat into and out of the metal hydrides. Heat is generated as the alloy absorbs and reacts with hydrogen. Heat must be added in order to release the hydrogen from the alloy. Unfortunately, most conventional designs for heat exchangers are not acceptable because either they (1) fail to allow for the expansion of the hydride powders or (2) they actually aggravate this problem. Tubes, fins, plates and the like which are the tools of heat transfer, can cause special problems inside hydride containers by impeding the motion of the powder as it expands during absorption of hydrogen. These objects may be strained, displaced or broken by the virtually irresistable forces from the expanding hydride.
As mentioned, hydride particles have no definable shape and their outside surface is characterized by jagged fractures in random directions. Therefore, movement and rearrangement of the powder is limited by the natural interlocking tendency of the hard, brittle particles. In other words, the particles lock together and exhibit poor flow characteristics.
The motion and rearrangement of the powder as it expands is further impeded by the problem of powder compaction, which is a result of spatially non-homogeneous reactions. Within a mass of hydride powders, the absorption of hydrogen is governed by the applied hydrogen pressure and temperature of the metal particles. Since metal hydrides are typically fine powders, gas flow within a volume of powders is restricted. In addition, metal hydrides are generally poor conductors of heat; thus, particles farthest from the heat-sink will have a limited ability to release heat and will react more slowly than those particles nearest the heat sink.
The result of either of the above will be the appearance of one or more reaction fronts which move through the powder with a definable direction. Those particles which are first to absorb hydrogen will tend to expand in the direction of least resistance within the container. Subsequent movement towards open space is impeded by inter-particle friction, gravity and by any obstructions such as fins or tubes inside the container. The expanding powder will also tend to compact those particles not yet reacted, i.e., the powder preceding the reaction front.
Associated with this powder compaction is the change in the bulk density. Those particles among the last to react will be subjected to high compacting pressures which aggravate the particle interlocking process. Within the hydride container, the last powders to react are both densified and impeded from rearranging themselves. Expansion of this powder may result in straining of the container walls.
As the hydrides desorb hydrogen, the particles contract, and any residual compacting pressure will only be due to the effect of gravity, which is small. However, and more importantly, the density gradients which occurred during the absorption of hydrogen may remain even after all the hydrides have desorbed gas. Successive absorption-desorption cycles can lead to a cummulative densification problem, which in turn causes a growth in area and force applied to the container walls and eventually, possible bursting of the container.
Because of these stress characteristics of sorbent alloys, care must be taken in the internal placement of heat transfer and gas dispersion devices in the storage container.
In U.S. Pat. No. 4,036,944, entitled "Hydrogen Sorbent Composition and Its Use", the combination of a metal hydride such as the alloy lanthanum pentanickel (LaNi.sub.5) with a thermal plastic elastomer binder is set forth. In the preferred embodiment, 2-15% by weight binder and 98-85% by weight sorbent (i.e., metal hydride) is utilized. Particles of the sorbent and binder are co-mingled, heated to the softening point of the binder, and molded into pellets. It was found in the U.S. Pat. No. '944 patent that the resulting pellet did not diminish the capacity of the sorbent to absorb and desorb hydrogen. In one embodiment wherein the pellets were 97% by weight LaNi.sub.5 and 3% by weight binder, a reduction in the attrition (i.e., disintegration of the particles into smaller sizes) was significantly reduced over using simply the sorbent particles.
In U.S. Pat. No. 4,135,621 issued to Turillon et al and entitled "Hydrogen Storage Module", a horizontal storage container is disclosed having a number of flutes formed around the circumference of an elongated metal cylinder to enhance heat transfer, and to aid in stacking, and wherein the ends are crimped over a gas-permeable filter disk. The interior of the cylinder is filled to a volume no greater than 78% of the total volume with a metal hydride.
In U.S. Pat. No. 4,134,491 issued to Turillon et al and entitled "Hydride Storage Containment", the invention teaches the use of at least one collapsible structure wtihin the hydrogen storage container wherein the collapsible structure may be permeable to hydrogen gas but impermeable to solids such as the hydrides. In the preferred embodiments of that invention, an elongated cylinder having a number of holes formed in the cylinder is utilized, a number of hollow capsules, spheres, or the like may be intermixed with the metal hydrides, or a number of foam-like objects such as cubes can be dispersed throughout the powder. These collapsible structures are used, preferably near the bottom of the container where densification is the greatest to aid in minimizing the deformation of the container after a number of cycles.
In U.S. Pat. No. 4,134,490 issued to Turillon et al and entitled "Gas Storage Containment", crumpled pieces of metal foil or fragmented turnings are intermixed with the metal sorbents in order to provide free space for expansion of the metal absorbents. Again, the expressed purpose for adding such second particulates is to minimize the expansion and possible rupture problem caused by the metal hydrides.
In U.S. Pat. No. 4,133,426 issued to Turillon et al and entitled "Hydride Container", a number of small storage capsules are utilized to store the metal sorbents. Each storage capsule has at least one end in communciation through a porous filter so that hydrogen gas can enter and leave the capsule. In the preferred operation, a large number of these individual storage capsules are then stacked inside a larger hydrogen storage unit. In the preferred embodiment, each capsule is 76.2 mm long and 12.2 mm in outside diameter having an internal volume of 9 cc. Again, these smaller cylinders are to be stacked in a horizontal position to minimize the compaction problem by maximizing the free gas space above the hydrides.
As can be witnessed from the above, the crux of the problem pertains to the limited ability of the sorbent material to flow or move within the container during expansion, and after contraction to relieve densification gradients. Within the field of material handling, the addition of "flow-aids" such as "lubricant" to powders to enhance their flow through, for example, bins, is well known. It has now been discovered that in the present invention the addition of a lubricant significantly reduces the results of expansion of hydrides in storage containers.